The invention relates to a shaft kiln for burning a mineral charge, particularly small-sized limestone, in a parallel-flow-regenerative process, with at least two circular cylindrical shafts, each of which has a preheating zone, a burning zone and a cooling zone and which are interconnected in the transition area between the burning zone and the cooling zone by a connecting passage for combustion gases flowing in periodically alternating directions, the shafts being surrounded by a common passage connected to the shaft areas, so that at the transition to the adjacent shaft, said passage forms the connecting passage.
A shaft kiln of this type corresponds to the known MAERZ construction. The circular cross-section of his shafts is necessary for an adequately uniform heat distribution over the shaft cross-section, if the kiln is to be suitable for larger throughput capacities, as cannot be achieved in the case of rectangular shaft cross-sections. There is also a good heat distribution of the joint, surrounding passage, in that it permits a uniform drawing off of combustion gases from the burning zone of one shaft and a uniform supply thereof to the adjacent shaft. However, it make it necessary to increase the diameter of the shaft area in the upper region of the cooling zone compared with the diameter of the part of the shaft area of the cooling zone located above it. Thus, there is an increased conical taper in the direction of the shaft bottom opening, which is of a conventional nature for shaft outlets, silo outlets, etc.
For a uniform heat distribution and for preventing a clogging or blocking up in the vicinity of the passages, it is also necessary to downwardly limit the lump size of the material to be burnt, so that to obtain a satisfactory quality of the burned product it is recommended that the lump size distribution be in the range 30 to 150 mm. Material with a small particle size of less than 30 mm must be separated and processed in some other way, e.g. in a more complicated rotary kiln operating with a lower thermal efficiency. DE-A-1 252 850 admittedly already proposes a shaft kiln suitable for burning a material with a minimum particle size of 10 mm, but it is then necessary to accept constructional disadvantages, which prevented the practical realization of such a kiln.